The present invention relates to an image recording device and an image recording system having a plurality of laser beams (multi-laser beam).
An image-recording device using a laser beam has been widely used because it runs faster with a higher resolution than image-recording devices based on other technologies.
A conventional printer using one laser beam (laser printer) is disclosed in Japanese patent application laid-open publication No. Hei 8-310057 (1996). The printer utilizes features of continuously modulating laser intensities in the main scanning direction and controlling the quantity of attached toner by controlling laser intensities for high-resolution printing. These features eliminate and reduce the irregularities in slanted outlines of characters and images, which makes the printout images and characters smooth.
To run the laser beam printer faster, it is required to make the laser beam (the light beam of a laser) scan at a high speed in both the main scanning direction (horizontally) and in a subsidiary scanning direction (vertically).
These requirements may be accomplished by rotating a photosensitive drum (for vertical scanning) and a rotary polygon mirror (for horizontal scanning) at high speeds. However, the rotational speed of the polygon mirror of the fastest conventional laser beam printer using one laser beam has almost reached its limit. Therefore, a multi-beam method of causing two or more laser beams to scan simultaneously is used instead of increasing the rotational speed of the polygon mirror.
Most laser beam printers (particularly printing systems that may be easily affected by the environmental conditions such as in electrophotography) frequently employ a method of varying the pulse duration (width) of a laser drive signal by modulation (PWM), thus controlling the quantity of light (that is, controlling the print dot sizes by light control) for assurance of picture qualities and stability when they print out multi-level images having pixels whose dot sizes (image data) are multi-leveled (gradated).
There are two methods of generating this pulse-width-modulated laser drive signal (pulses): an Analog method of generating a device signal by comparing a triangular wave created in synchronism with image data by a D/A converter output of the image data, for example, as disclosed in Japanese patent application laid-open publication No. Sho 62-39972 (1987); and a Digital method of generating a drive signal logically (by frequency-division) from a fast clock whose frequency is 4 to 8 times as high as the image clock, as disclosed in Japanese application patent laid-open publication No. Hei 5-6438 (1993).
As described above, a fast printer system for printing multi-level images typically employs a multi-beam method using a pulse-width modulation technique.
Although a laser printer using a multi-beam method is disclosed in Japanese application patent laid-open publication No. Hei 8-15623 (1996), this method may reduce the image accuracy according to uneven dot sizes if the light quantities of the laser sources are not equal. To solve such a problem, a technique is proposed for correcting the light quantities of the laser sources.
For example, Japanese application patent laid-open publication No. Hei 5-212904 (1993) discloses a method of applying a driving signal of an identical pulse width to the driving circuit of each laser source which emits illuminating dots, measuring the intensity of each illuminating dot, and calculating light correction values from the measured intensities of light (light dispersion). This example calculates the ratio of the maximum value Xmax of the light quantity data to the minimum value Xmin, multiplies the image data L by the ratio, multiplies the product by a correction factor Xmin/X for each illuminating dot calculated from the light quantity data X and the minimum value Xmin, and thus obtains the corrected image data L.
There is disclosed another embodiment in Japanese application patent laid-open publication No. Hei 7-199096 (1995). The embodiment detects the quantity of laser light emitted from each laser source using a sensor, compares it to a preset target value, and controls the current of each laser source so that the quantities of laser lights from the laser sources become identical.